Hearing aids are electronic instruments worn in or around the ear that compensate for hearing losses by amplifying and processing sound. The electronic circuitry of the device is contained within a housing that is commonly either placed in the external ear canal or behind the ear. Transducers for converting sound to an electrical signal and vice-versa may be integrated into the housing or external to it.
Whether due to a conduction deficit or sensorineural damage, hearing loss in most patients occurs non-uniformly over the audio frequency range, most commonly at high frequencies. Hearing aids may be designed to compensate for such hearing deficits by amplifying received sound in a frequency-specific manner, thus acting as a kind of acoustic equalizer that compensates for the abnormal frequency response of the impaired ear. Adjusting a hearing aid's frequency specific amplification characteristics to achieve a desired level of compensation for an individual patient is referred to as fitting the hearing aid. One common way of fitting a hearing aid is to measure hearing loss, apply a fitting algorithm, and fine-tune the hearing aid parameters.
Hearing loss is measured by testing the patient with a series of audio tones at different frequencies. The level of each tone is adjusted to a threshold level at which it is barely perceived by the patient, and the audiogram or hearing deficit at each tested frequency is quantified as the elevation of the patient's threshold above the level defined as normal by ANSI standards. For example, if the normal hearing threshold for a particular frequency is 4 dB SPL, and the patient's hearing threshold is 47 dB SPL, the patient is said to have 43 dB of hearing loss at that frequency.
Compensation is then initially provided through a fitting algorithm. This is a formula which takes the patient's audiogram data as input to the formula and calculates gain and compression ratio at each frequency. A commonly used fitting algorithm is the NAL_NL1 fitting formula derived by the National Acoustic Laboratories in Australia and the DSL-i/o fitting formula derived at the University of Western Ontario. The audiogram provides only a simple characterization of the impairment to someone's ear and does not differentiate between different physiological mechanisms of loss such as inner hair cell damage, as opposed to, outer hair cell damage. Patients with the same audiogram often show considerable individual differences, with differences in their speech understanding ability, loudness perception, and hearing aid preference. Because of this, the initial fit based on the audiogram is not usually the best or final fit of the hearing aid parameters to the patient. In order to address individual differences, fine-tuning of the hearing aid parameters is conducted by the audiologists.
Typically, the patient will wear a hearing aid for one-to-three weeks and return to the audiologist's office, whereupon the audiologist will make modifications to the hearing aid parameters based on the experience that the patient had with real-world sound in different environments, such as in a restaurant, in their kitchen or on a bus. For example, a patient may say that they like to listen to the radio while washing dishes, but with the hearing aid loud enough to hear the radio, the sound of the silverware hitting the dishes is sharp and unpleasant. The audiologist might make adjustments to the hearing aid by reducing the gain and adjusting the compression ratio in the high frequency region to preserve the listening experience of the radio while making the silverware sound more pleasant. Whether these adjustments solve the problem for the patient, however, will only be determined later when the patient experiences those problem sounds in those problem environments again. The patient may have to return to the audiologist's office several times for adjustments to their hearing aid until all sounds are set appropriately for their impairment and preference.
This process could be improved if the audiologist were able to create a real-world experience so that the patient could instantly tell the audiologist if the adjustments that are made are successful or not. In the above example, if the audiologist could present the real-world sounds of a radio and a fork on a plate while washing dishes to the patient, the audiologist could make as many adjustments as necessary to optimize the hearing aid setting for that sound during a single office visit, rather than having to make an adjustment, have the patient go back home and experience the new setting, then come back to the office if the experience wasn't optimal.
To address this problem, some hearing aid manufacturers have provided realistic sounds in their fitting software that use a 5.1 surround speaker setup. The surround sound is important because the spatial location can affect the sound quality and speech intelligibility of what they hear. Without it, the fine-tuning adjustments made in the audiologist's office may not be optimal for the real world in which the patient experiences problems. Also, natural reverberation, a problem sound for hearing aid wearers, is better reproduced with surround speakers than with a typical stereo front-placement speaker setup. Unfortunately, most audiologists' offices do not have 5.1 surround speaker setups, either due to cost, space, lack of supportive driving hardware, unfamiliarity with setup and calibration, or multiples of the above.
Spatial hearing is an important ability in normal hearing individuals, with echo suppression, localization, and spatial release from masking being some of the benefits provided. Audiologists would like to be able to demonstrate that hearing aids provide these benefits to their patients, and this can be done with a surround speaker setup but not the typical two-speaker stereo setup that exists in most clinics. Any hearing aid algorithms that were developed for these spatial percepts will be difficult to demonstrate in the audiologist's office.